In the meantime, I’ve found a workaround. In addition to the NVIDIA GPU (RTX 4060), I can use the onboard graphics via the Intel CPU. I hadn’t thought of that at all before.
So I “hide” the NVIDIA GPU during the initial installation of Qubes 4.3 by manually modifying the GRUB boot parameters and adding:
i915.force_probe=a780 i915.enable_psr=0 rd.qubes.hide_pci=01:00.0,01:00.1
And just like that, I can install 4.3 without any problems.